Technique for generating planar beams from a linear doppler line source employing a circular parallel-plate waveguide

ABSTRACT

A circular parallel-plate wave guide converter applied to the array of a linear Doppler-scanned antenna or to the array of an electronic-scanned antenna, to convert either of those antenna systems, which normally radiate conical-coordinate beams, to a system which radiates planar-coordinate beams, in the former case coded as to angle of radiation as a function of frequency. In one form, the aperture of the plate system comprises a 180 degree horn formed by flaring the circular perimeters of the plates. In another form, the horn section is folded downward to provide illumination as an offset feed for a 180 degree (in azimuth) reflector surface which is circular and convex in azimuth and also concave outward in any vertical plane.

United States Patent 191 Nemit Nov. 27, 1973 TECHNIQUE FOR GENERATING PLANAR BEAMS FROM A LINEAR DOPPLER LINE SOURCE EMPLOYING A CIRCULAR PARALLEL-PLATE WAVEGUIDE [75] Inventor: Jeffrey T. Nemit, Los Angeles, Calif.

[73] Assignee: International Telephone and Telegraph Corporation, New York, N.Y.

[22] Filed: July 1771972 [21] Appl. No.: 272,451

[56] References Cited UNITED STATES PATENTS 10/1955 ll/l953 Spencer 343/786 Gruenberg 343/786 Primary Examiner-Eli Lieberman Attorney-C. Cornell Remsen, Jr. et al.

[ ABSTRACT A circular parallel-plate wave guide converter applied to the array of a linear Doppler-scanned antenna or to the array of an electronic-scanned antenna, to convert either of those antenna systems, which normally radiate conical-coordinate beams, to a system which radiates planar-coordinate beams, in the former case coded as to angle of radiation as a function of frequency. In one form, the aperture of the plate system comprises a 180 degree horn formed by flaring the circular perimeters of the plates. In another form, the horn section is folded downward to provide illumination as an offset feed for a ISOldegree (in azimuth) reflector surface which is circular and convex in azimuth and also concave outward in any vertical plane.

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26 aa e4 TECHNIQUE FOR GENERATING PLANAR BEAMS FROM A LINEAR DOPPLER LINE SOURCE EMPLOYING A CIRCULAR PARALLEL-PLATE WAVEGUIDE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to antenna systems for electromagnetic radiation and reception and, more particularly, to the generation of planar beams from a time-modulated array. I

2. Description of the Prior Art In the prior art, there are many applications in direction finding and radio navigation for both Doppler-scan techniques and for electronic-scan phased-array systems. Both of these general types of scanning antennas produce beams which are inherently describable in terms of conical coordinates. Mechanically rotating antennas, on the other hand (such as in the usual PPI systems), typically produce beams in planar coordinates. In many applications it is desirable, or even necessary, that bearing information determined by a radar system be provided in planar coordinates. Accordingly, since these aforementioned electronic-scanning techniques typically provide bearing information in conical coordinates, a need exists for a simple technique for generating planar beams from a linear Doppler line source, or linear phased array. The manner in which the present invention solves this problem will be evident as this description proceeds.

The so-called Doppler-scan technique, to which the present invention is applicable, is described in the technical and patent literature. For example, U. S. Pat. No. 3,61 3,096 describes a typical Doppler nagivation system. U. S. Pat. No. 3,670,338 and U. S. application Ser. No. 210,699, now US. Pat. No. 3,728,729, filed Dec. 22, 1971, are useful background information in that they show aspects of course path control and ways in which the program of element commutation or successive excitation can be varied for system purposes, respectively.

The general subject of phased arrays is also described in the technical and patent literature. Chapter 11, entitled Array Antennas," of Radar Handbook by Merrill Skolnik (McGraw-Hill Book Co., 1970) is devoted entirely to the subject of array antennas and extensively describes the state of the art in respect to phased arrays and phased-array systems. This reference material is of interest in that it describes the second general type of linear antenna array with which the present invention is useful.

In the so-called Doppler navigational system, the concept of commutating the elements of a linear array generally transversely with respect to the approach path of an aircraft (on landing approach, for example) is relied upon to produce an apparent Doppler component on the transmitted signal received by the said aircraft on approach. A source of frequency f moves along a linear path at velocity v along the length (or aperture) of the array. The signal observed at point p at an angle 4), for a single scan of the source across the aperture is:

A (x) amplitude of radiated signal x V I As the above equations indicate, the received signal includes a Doppler component or shift, which is proportional to the scan velocity and the sine of the observation angle. The measurement of this Doppler shift in frequency provides bearing angle information to the observer (the approaching aircraft) with respect to the normal of the said Doppler antenna array. To facilitate measurement, a reference signal is typically radiated from an auxiliary antenna adjacent to the Doppler antenna. This reference signal source and the measurement system, per se, are not a part of the present invention and are omitted from description herein, although the above-referenced prior art patent literature does describe the system aspects of such Doppler navigational systems.

From the above mathematical expressions, it will be apparent that the surface of constant Doppler shift takes the shape of a cone with the linear array as its axis. Bearing information is, therefore, provided in conical coordinates. This is analogous to the situation in which a linear phased-array antenna provides a conical fan beam. FIG. 1 of the drawings herewith is included to illustrate this situation graphically. The antenna array itself extends from -L/2 to L/2 in that figure.

In many applications it is desirable, or even necessary, that the bearing information be provided in planar coordinates for system reasons. Stated otherwise, it may be said that it is desirable that bearing information be provided in a manner comparable to that provided by a rotating antenna PPI system: i.e., in planar coordinates. The manner in which the transformation of inherently conical coordinate array systems is made to operate in planar coordinates, according to the present invention, is hereinafter described.

SUMMARY OF THE INVENTION This specification describes a new technique for providing an encoded signal in space in planar coordinates. By encoded, in this sense, we mean that it is described by a frequency characteristic at each angle, discretely. The system is most useful for azimuth or bearing determinations, and accordingly, will be described in that context. As hereinbefore indicated, the invention may be applied to either Doppler or electronic-scanning antenna arrays to perform the conversion to planar coordinates.

In accordance with the invention, a horizontal linear array feeds into a pair of semi-circular horizontal plates spaced in accordance with general criteria for propagation in wave guide. The semi-circular perimeter of these plates represents the output or aperture of the device and may include flaring to provide, in effect, a degree sectoral horn. In another embodiment, this horn aperture is turned under so that it illuminates a 180 degree complex reflector placed preferably (but not necessarily) below the plates. The reflector, although convex outward in the aximuth plane, is concave in any elevation or vertical plane intersecting it. The horn aperture of the plates acts as an offset feed for this reflector.

The horizontal linear array is located in the plates essentially along the straight side, which would be a diameter. if the plates were full circles. The details and mathematical justification for the results produced are hereinafter set forth in the description of the preferred embodiments.

It may be said that the general object of the present invention is the provision of means for converting an inherently conical-coordinate array system to one operating in planar coordinates for the provision of bearing information.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of basic Doppler scan technique (prior art).

FIG. 2 is a geometrical diagram comparing planar and conical coordinates for an azimuth or bearingdetermining antenna.

FIG. 3 is a plan view of a parallel-plate wave guide arrangement to convert conical beams to planar beams for a Doppler-scan system in accordance with the present invention.

FIG. 4a is a further detail of the arrangement of FIG. 3.

FIG. 4b is a semi-pictorial view into the aperture of the arrangement of FIG. 4a, showing the 180 degree sectoral horn.

FIG. 4c is an offset feed and reflector version of the circular-plate coordinate converter.

FIG. 4d illustrates a detail of array coupling into the circular-plate structure in accordance with section AA of FIG. 4a.

FIGS. 4e and 4f illustrate two forms of slotted-array feed adaptable to the structure of FIG. 4a or FIG. 4c as viewed in accordance with section BB of FIG. 4a.

FIG. 5 illustrates the use of the parallel plates of the present invention with an electronic-scanning antenna, such as a linear phased array.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, the basic notations in FIG. 1, hereinabove referred to, will be used throughout the remainder of this description.

FIG. 2 illustrates the same concepts as FIG. 1, and is self-explanatory. FIG. 2 depicts graphically the difference, or relationship, between conical coordinates and planar coordinates for a horizontal azimuth (bearing determining) antenna system.

To simplify the discussion of a planar-beam Doppler antenna system, it is helpful, according to the invention, to express the aperture excitation in a form somewhat different than hereinbefore given. First, it will be shown that a moving source, which is periodically scanned, can be decomposed into a series of finite plane-wave components. Each component is at a different frequency and results in a beam in a different direction. The action of the parallel-plate wave guide on each of these components will then be shown to convert the normally conical beams to planar beams. The radiation pattern of a moving source which is periodically scanned, and as we would now choose to express it, is equal to:

The net result of the foregoing is a series of beams of different angles, each at a different frequency. Note also that:

F (u) Field pattern of aperture distrobution A (x) Associated with each beam is a distinct excitation 5,, (x), at a specific frequency:

The decomposition of the excitation into orthogonal components provides a simple means for explaining the operation of the planar-beam Doppler antenna illustrated in FIG. 3. That figure is a plan view of a parallelplate wave guide for the azimuth antenna configuration being considered. Two spaced parallel plates providing essentially a degree aperture, having a radius R,,, and fed by a Doppler scanning array as a feed mechanism 1, and having a length L, are shown. A source which periodically traverse (by commutation) the aperture of the feed mechanism 1 over the length dimension L is enclosed in the parallel-plate region essentially along what would be a diameter of a circle of which the semi-circular parallel plates (typically 2) would be a part. The plate spacing is determined by the polarization desired and the frequency of radiation. A plate spacing of less than one-half wavelength would be appropriate for vertical polarization in space, whereas a spacing lying in the region of one-half to one full wavelength would be appropriate for horizontal polarization in space. Although it would be possible to operate with place spacings outside of those preferred regions, a plate spacing much below one-quarter wavelength is likely to unnecessarily complicate the transition problem.

Consider next the Doppler feed excitation S,, (x) at a frequency To determine the resultant radiation pattern, it is first necessary to determine the excitation on the cylindrical aperture. Since the aperture is large in wavelengths, a geometric-optics approximation can be employed and the cylindrical aperture excitation can be determined on the basis of rays, as illustrated in FIG. 3. Rays from the end of the Doppler scanning feed mechanism, identified as 3 and 4, demark a cord identified as L cos @n. The resultant cylindrical aperture excitation is:

e j o o cos bit) The foregoing describes essentially the same excitation as would have to be impressed on the cylindrical aperture to provide a beam at I Note that the center of the excitation on the cylindrical aperture is at:

If a different Doppler beam, m, is selected, then the excitation is centered at:

The important ramification of this discussion is that the radiating excitation moves around the cylindrical aperture in synchronism with the beam direction. This is a basic requirement for generating a planar beam with a cylindrical antenna. Thus, the result obtained is analogous to a mechanically scanned antenna; e.g., one of the PPI-type rotating about a common axis.

A characteristic of the configuration of FIG. 3 is tha it exhibits a broader azimuth beam width at increasing azimuth angles, as well as the defocusing in elevation which is characteristic of cylindrical antennas. This beam broadening with azimuth angle is caused by the varying effective aperture size with azimuth angle. The defocusing of the beam with elevation angle is caused by the curvature of the cylindrical aperture, and can be minimized by selecting a radius, R, for a given aperture size, L, to minimize S. In addition, the Doppler feed excitation can be slightly defocused to minimize the maximum defocusing or to compensate therefor in the elevation coverage region. In FIG. 4a the parallel plates are identified as 2 with 5 under it. A flared section comprising 2' and 5 forms a curved aperture horn which may be covered with a radome, if desired. FIG. 412 provides a view looking into the aperture of the FIG. 4a arrangement.

A more practical arrangement in terms of performance and enlargement of the vertical aperture (as compared to FIG. 4a) is depicted at FIG. 40. Here the same general plate configuration, comprising plates 2 and 5 in the same shape as generally depicted in FIGS. 3 and 4a, is provided. In FIG. 40, however, the flared sections depicted as 2" and 5" are curved under by an angle somewhat in excess of 90. This produces a downturned sectoral horn covering the same 180 degree azimuth and which acts as an offset feed for a reflector 6. This reflector 6 is substantially semi-circular (convex outward) in any azimuth plane passing through it, but it is concave outward (parabolic, for example) in any vertical plane passing through the center of the circular plates 2 and 5. An input 7 connected to the feed array 1 operates as it would in connection with FIG. 4a. An RF source is commutated by a commutator 21 producing the traveling excitation point along the array 1 in accordance with the Doppler navigation system prior art.

A characteristic of the configuration of FIG. 40 is that a broader azimuth beam width at increasing azimuth angles is obtained. However, the defocusing in the elevation plane which is characteristic of cylindrical antennas is significally reduced. The beam broadening with azimuth angle is caused by the variable effective aperture size as a function of azimuth angle. The

defocusing of the beam with elevation angle is caused by curvature of the cylindrical aperture, and its minimization is accomplished by selecting a proper radius, R, for a given aperture size, L, in order to minimize S. The reader may refer to the foregoing equations in order to further evaluate the significance of this statement. It is also possible to intentionally defocus the Doppler feed excitation slightly to minimize the maximum defocusing in the elevation coverage region.

FIGS. 40', 4e and 4f apply equally well to the configuration of FIGS. 4a and 40.

FIG. 4d illustrates the situation in which the wave guide plates 2 and 5 are fed from a series of probes, typically 15, introduced by a typical coaxial transmission line 14. A backwall closure 13 is required in this case. The flared sections of the plates 2 and 5, namely 2 and 5, are those previously discussed, or could be in accordance with 2" and 5", as indicated in FIG. 40. FIGS. 4e and 4f illustrate excitation of the space between the plates 2 and 5 from slots in an enclosure wall 13. Slots l8 and 19 are typical of those required for excitation to produce vertical polarization, whereas typical slots 16 and 17 of FIG. 4]" are those required to produce horizontal polarization.

Referring now to FIG. 5, the adaptation of the invention to the electronic phase scanning case is illustrated. Typical transitions 8 and 9 might be any one of the types illustrated in FIGS. 4d, 4e or 4f. Each of these individual transitions are excitation elements fed through a discrete phase shifter, typically 10 and 11, as illustrated. An azimuth distribution network 12 energizes the RF inputs of these controllable phase shifters sub stantially uniformly. This type of controllable phase shifter beam-pointing system is well known of itself, the aforementioned radar handbook text and the patent literature, in general, providing background information as to the nature of these phase shifters and their control and programming in such a system. A duplexer 22 makes possible the operation of this form of the system with RF source 23 and a receiver 24 in a manner well understood in this art.

It will be understood, also, that the semi-circular plate 2 in FIG. 5 and plate number 5 concealed beneath it might be in accordance with either the FIG. 4a or 40 configurations.

It will be realized that certain well-known microwave instrumentation techniques and details are readily recognized and inferred from the drawings by those skilled in this art. For examples, the plates 2 and 5, and the horn sections into which they are flared, are made of conductive material, usually thin conductive metal sheets. It is not considered necessary to recite the details of materials, insulations, etc., in this application, such matters being obvious to those skilled in this art.

Modifications and variations on the embodiments shown and described will suggest themselves to those skilled in the art once the principles of the invention are understood. Accordingly, it is not'intended that the scope of the invention should be limited to the structure illustrated and this description, these being typical and illustrative only.

I claim:

1. An antenna system for radiating planar beams coded as to angle of radiation by frequency as determined at a remote receiving station, comprising:

a linear array of radiating elements disposed along a first spatial coordinate;

a pair of spaced conductive plates of generally circular outline each lying in a plane parallel to a plane containing the line of said array along said first spatial coordinate, said plates being spaced to act as a waveguide;

and means for locating said array between and substantially along a line parallel to a pair of parallel diameters of said plates, to provide excitation of the space between said plates as a waveguide, the open edges thereof forming a circular sector aperture, said radiation from said aperture providing said planar beams as compared to the conical coordinates inherent from use of said array alone.

2. Apparatus according to claim 1 in which saidcircular sector is substantially 180.

3. Apparatus according to claim 2 in which said parallel plates are flared to form a horn aperture extending around said circular sector.

4. Apparatus according to claim 3 in which said array comprises a plurality of substantially equally spaced, independently excitable radiating elements, an RF source and commutating means whereby said elements may be excited from said RF source in accordance with a predetermined commutation pattern.

5. Apparatus according to claim 3 in which said array comprises a plurality of spaced radiating elements fed from an azimuth distribution network, each through a discrete corresponding phase-shifting means to form a planar beam which is a function of the distributed phase-shift program among said phase-shifting means.

6. Apparatus according to claim 3 in which the planes of said plates are substantially horizontal, whereby said aperture is a cylindrical sector curved in the horizontal plane.

7. Apparatus according to claim 3 including a reflector having a shape which is convex outward in the azimuth plane and concave outward in all vertical planes intersecting said reflector, in which said plates are vertically displaced with respect to said reflector, and in which said plates are curved toward said reflector such that said horn aperture forms an offset degree sectoral horn feed for illuminating said reflector.

8. Apparatus according to claim 6 in which the spacing of said plates is predetermined, in the unfiared portion, for vertical polarization operation at a value less than one-half wavelength and at a value between onehalf and one full wavelength for horizontal polarization operation.

9. Apparatus according to claim 7 in which the spacing of said plates is predetermined, in the unflared portion, for vertical polarization operation at a value less than k wavelength and at a value between one-half and one full wavelength for horizontal polarization operation.

10. Apparatus according to claim 3 in which the spacing of said plates is predetermined, in the unflared portion, for vertical polarization operation at a value less than k wavelength and at a value between one-half and one full wavelength for horizontal polarization operation. 

1. An antenna system for radiating planar beams coded as to angle of radiation by frequency as determined at a remote receiving station, comprising: a linear array of radiating elements disposed along a first spatial coordinate; a pair of spaced conductive plates of generally circular outline each lying in a plane parallel to a plane containing the line of said array along said first spatial coordinate, said plates being spaced to act as a waveguide; and means for locating said array between and substantially along a line parallel to a pair of parallel diameters of said plates, to provide excitation of the space between said plates as a waveguide, the open edges thereof forming a circular sector aperture, said radiation from said aperture providing said planar beams as compared to the conical coordinates inherent from use of said array alone.
 2. Apparatus according to claim 1 in which said circular sector is substantially 180*.
 3. Apparatus according to claim 2 in which said parallel plates are flared to form a horn aperture extending around said circular sector.
 4. Apparatus according to claim 3 in which said array comprises a plurality of substantially equally spaced, independently excitable radiating elements, an RF source and commutating means whereby said elements may be excited from said RF source in accordance with a predetermined commutation pattern.
 5. Apparatus according to claim 3 in which said array comprises a plurality of spaced radiating elements fed from an azimuth distribution network, each through a discrete corresponding phase-shifting means to form a planar beam which is a function of the distributed phase-shift program among said phase-shifting means.
 6. Apparatus according to claim 3 in which the planes of said plates are substantially horizontal, whereby said aperture is a cylindrical sector curved in the horizontal plane.
 7. Apparatus according to claim 3 including a reflector having a shape which is convex outward in the azimuth plane and concave outward in all vertical planes intersecting said reflector, in which said plates are vertically displaced with respect to said reflector, and in which said plates are curved toward said reflector such that said horn aperture forms an offset 180 degree sectoral horn feed for illuminating said reflector.
 8. Apparatus according to claim 6 in which the spacing of said plates is predetermined, in the unflared portion, for vertical polarization operation at a value less than one-half wavelength and at a value between one-half and one full wavelength for horizontal polarization operation.
 9. Apparatus according to claim 7 in which the spacing of said plates is predetermined, in the unflared portion, for vertical polarization operation at a value less than 1/2 wavelength and at a value between one-half and one full wavelength for horizontal polarization operation.
 10. Apparatus according to claim 3 in which the spacing of said plates is predetermined, in the unflared portion, for vertical polarization operation at a value less than 1/2 wavelength and at a value between one-half and one full wavelength for horizontal polarization operation. 